I have a very fragmented world, composed of different kingdoms/nations most of which have no connections with the others. How can I explain the adoption of the same calendar (or metric system for that matter) at a certain point in the past without using a common root civilization that conquered all of them?

The calendar in question doesn't have to be too complex, no leap years are needed for example. But still when I mean "same" I mean technically the same, but not culturally.

So same number of months, same number of days per month, same number of days per week. Names for months and days can vary depending on the culture.

Also I'm not saying no contact has ever happened, but the contacts were scarce. No cultural invasion.

  • $\begingroup$ They have to have some connection humans have to come from somewhere, and if they are connected by land the changes that they have never traded will be very very small. $\endgroup$
    – John
    Sep 12 '18 at 21:31
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  • $\begingroup$ I don't get why this is problematic. In the 14th century England and Russia used the same (well, almost the same) calendar, without ever having been conquered by the same "root civilization". And, in our very days, China and Paraguay use the same calendar. A good calendar is hard to find; once found, it will spread just as any other cultural achievement. $\endgroup$
    – AlexP
    Sep 12 '18 at 23:58
  • $\begingroup$ Strong biorhythms? Can your people have regular innate rhythms in which they sleep, eat, grow, hibernate? $\endgroup$ Sep 13 '18 at 0:01
  • $\begingroup$ @AlexP both england and russia have been "conquered" by christianity, and that's where the calendar comes from. china adopted the western calendar only very recently. the chinese calendar is still in use and defines national holidays, birthday celebrations, etc... $\endgroup$
    – eMBee
    Sep 13 '18 at 3:43

It is based in some regular, useful, synchronized, easily recognizable phenomena

In Earth, calendars based in the lunar cycle are common: it is easy to look up during the night and see that the moon now is no exactly like yesterday, and a lot like it was a month before.

So, you need something in your world that is regular enough to allow to use as a measure of time and , global for everyone to notice. Here the most plausible candidate is again astronomy.

In this case, you may want to avoid the issue that caused the downfall of the lunar calendaries: lunar cycles do not match solar cycles, and solar cycles:

A) Are necessary to keep track of if you want to develop agriculture / predict weather.

B) Being longer, induce a greater possibility of error in its calculations.

Make your moon perfectly synchronized with your solar cycle, so that each year are exactly (or close enough to not be relevant to primitive cultures) X months. Additionally, you could try to add a second satellite with a different cycle (but still synchronized with the solar one) so combination of the two moons'states are useful to track time in the year (e.g., twice each year the two moons will be full moons, which mean that you can use it to mark the start of the seasons).

  • $\begingroup$ I was going to answer the same thing. The benefit of an astronomical basis of timekeeping (especially one based on the sun and moon, rather than the stars) is that it's global: different cultures can and will make note of the same useful coincidence, arriving at basically the same calendar. $\endgroup$
    – Cadence
    Sep 12 '18 at 21:58
  • $\begingroup$ I think you make good points for the ancient invention of calendars. But as I read the original query, the OP assumes that ancient calendars based on lunar and solar cycles already exist in the forms of the native calendars of these "fragmented" countries and civilisations. They're asking how can a calendar be adopted at a later time and around the world. I had to down-vote because you didn't actually answer the question! $\endgroup$
    – elemtilas
    Sep 13 '18 at 0:05
  • 2
    $\begingroup$ @elemtilas The point is that it needn't be the same calendar. If the astronomical basis is clear enough, different civilizations will come to the same conclusions independently. When they do find each other, their calendars will already be near enough to identical to fit with no problem. $\endgroup$
    – Cadence
    Sep 13 '18 at 1:56
  • $\begingroup$ @Cadence -- I don't understand what you're getting at here... All (Earth) calendars are ultimately astronomically based: whether it's planting seasons or moon cycles or sun cycles or star cycles. Like I said, you made good points about the origins of calendars. But that's not the question. The OP, as best I can figure, is asking how a number of disconnected civilisations can all adopt the same calendar. It's not said, but I can only presume that by the time of the adoption, all these cultures already have their own calendars. And they may all be somewhat similar! $\endgroup$
    – elemtilas
    Sep 13 '18 at 4:09


Even rare trade would have an influence on the way people measure anything. After all, you need to deal with the proverbial pound on one side and kilogram on the other. Traders become experts in conversion and the greatest evangelists of standardized systems.

Believe you me, the last thing a trader from Űryk wants to do is arrive on the last day of the Improvidence of Kœđđna during the Descension of Botath Reporatory — lest he become part of the Expression of Union.

You betcha, traders want everyone using exactly the same calendar.

  • 3
    $\begingroup$ I think you're nailing the proverbial screw on the threads here. TRADE is the right answer. Just look to the primary world and the worldwide gregorian calendar. Its origins are irrelevant, though interesting. What's key is how it spread. Its use spread into the Americas along with the Spanish, French and English empires, where it ousted the Native calendars almost entirely. Same goes for subsaharan Africa. In the Levant & Asia, its use has not supplanted the ancient calendars, but for matters of commerce, science and civil intercourse, its dominance can not be ignored. (cont.) $\endgroup$
    – elemtilas
    Sep 12 '18 at 22:30
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    $\begingroup$ And the dominant factors in commerce, science, culture, etc. since the 16th century has been Western Europe and in particular the Empire (now Commonwealth) and at last the USA, the economic heir the British Empire. It comes as no surprise that the civil calendar of the West has been so widely adopted, and, perhaps, along with some other factors (such as language & scientific method, etc) will outlast the USA as dominant force, should that event occur. So, +1 for actually answering the question! $\endgroup$
    – elemtilas
    Sep 12 '18 at 22:34
  • $\begingroup$ @elemtilas Except that trade in Earth has not produced such effect until recently (most of your examples are not of trade but of colonialism/conquest). If your travel can last from one week to one month, depending on how nice the weather is, if you find some caravan going your way, your ship is not hit by a storm, etc...., having a precise calendar is not very helpful. $\endgroup$
    – SJuan76
    Sep 12 '18 at 23:02
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    $\begingroup$ @SJuan76, The Gregorian calendar was created over 400 years ago and spread through conquest, diplomacy, and trade. It exists to fix the date of Easter, regardless of weather, and traders had to meet that deadline, weather delays or not (making calendars and timekeeping very important). Perhaps ancient traders (thousands of years ago) lived imprecise lives, but even they had to worry about the timing of harvests and festivals. Methinks you need to provide citations for your concerns because I don't think they're as real as you believe. $\endgroup$ Sep 12 '18 at 23:14
  • $\begingroup$ You do not provide citations for your assertions yet I do have to provide for mine? Well, for example, simply the existence and abundance of shipwrecks tells you that going from point A to point B was not as easy as you make it seem. If you had bad weather, you waited until it was ok, and until the damage to your ship was repaired. Or you could read about Marco Polo travels. Or other medieval embassies. Your references to festivals are ok only for very local trade, but that would have not spread your calendar too far away and the premise of the OP is that the countries have no contact. $\endgroup$
    – SJuan76
    Sep 12 '18 at 23:48

One option not previously mentioned is a former world encompassing empire. The empire may have broken up so long ago that it is just a myth but some things will be retained like the calendar format and possibly measuring units.

There may be other cultural parallels like similar deities and take your hat off in buildings.

Some empires allow different regional languages (so long as official business is done in the empire's language). So, if the empire's language in remote areas was only known to a few, after the breakup, only the local language is likely to survive.

Reasons for the breakup could be:

  1. Some external threat: Someone/thing/group from outside the empire destroyed it well enough that the survivors had to begin from scratch. I would also include disease or plague in this category. What was the threat? Is it still out there? will it be back? Why did it leave after breaking up the empire?
  2. Succession issues: People pick sides and fight it out first with words and then with property damage and lives lost. The empire could have just worn itself away through a series of battles until it didn't have any interest in its vassals. In this case the fighting heads toward the center of the empire with fewer and fewer troops available. Think of two very tired and beat up boxers who keep standing there, throwing punches at each other.
  3. Rebellion: The vassal states have had enough or a few governors get greedy and it becomes the empire against everyone. In this scenario, the time frame can be the shortest as each group does its best to erase the empire from all records and tales.

Then you need to decide what is left of the empire and why people don't see it. Was it all looted and pillaged? Is there disease or poison in a "forbidden zone"? Is there some kind of automated defense in the Forbidden zone? Is there a rocky field of craters where the seat of the empire once stood (or an ocean)?

  • 3
    $\begingroup$ Another good response that answers the question! And, at our point in primary world history, very much mimics the historical situation! For the vast majority of people on Earth, the idea of a world-spanning empire is all but mythological, and is just barely historical. I concur: such an empire could have this effect. And historically, we've seen it in action: the Roman Empire left behind its language and to an extent its civil governance and law which would greatly influence later European countries who had no direct connection to the Empire. Also, the British Empire's legacy around the world. $\endgroup$
    – elemtilas
    Sep 13 '18 at 0:11
  • $\begingroup$ Rome was the first thing that came to mind. The British Empire never occurred to me. Probably because it is too recent and is still part of recorded history. $\endgroup$
    – ShadoCat
    Sep 13 '18 at 0:44
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    $\begingroup$ Well, Rome is too! But since the OP doesn't specify a time-frame, I figure either would work! For what it's worth, although we call it "Gregorian", the calendar we actually use is Pope Gregory's reform of the Julian calendar, was the brainchild of G. Julius Caesar, itself a reform of the earlier Roman calendar. So in a sense, even though it was Spain, France and GB that did most of the spreading in the last 500 years, it was spread to them at an earlier time and in an earlier form. $\endgroup$
    – elemtilas
    Sep 13 '18 at 4:04
  • $\begingroup$ @elemtilas, that makes sense. You are right, the OP didn't specify that the other civilizations couldn't know each other or the time frame of the adoption. $\endgroup$
    – ShadoCat
    Sep 14 '18 at 17:09

They may reach the same conclusion if the observable phenomena is similar.

  • A plant that blooms and withers each month.
  • A wasp molting each week.
  • A tree giving fruit each year.

The wasp pollinates the plant, which in turn is a symbiotic organism on the tree.

  • Tree - year
  • Flower - month
  • Wasp - week

There! You have your calendar!

  • $\begingroup$ Thx for the edit :) $\endgroup$
    – Gustavo
    Sep 13 '18 at 20:03

A regular eclipse

Will start everyone's "year"¹. This is an event this is super obvious and awe inspiring enough to make everyone revolve their year around that. Then they split their calendars into sets of 10, since they count in base ten, so that makes the most sense. It would be easier know how far days are away cross-month.

It would help greatly if the number of days in the year is an exact multiple of 100, since months can then be broken into days that are a multiple of ten. They all count in base ten because that's how many fingers they have.

¹: Not necessary a scientific definition of a year (1 full orbit around the planet's star) But it would help greatly if it were close, so the seasons will roughly align with their "year". Otherwise you can also have seasons not be bound to months, so people get the new date ranges for each season at the start of each year.

Mommy, when does summer start this year?

It starts at 3-6.7

Wow, that's only 50 days away! It's a good thing the months use the same base as our days or this would have been harder to figure out.



most major religions on earth defined their own calendars, and although historically the spread of religion was culturally invasive, it doesn't have to be. if the religion doesn't conflict with local cultural practices and doesn't introduce new rituals, it doesn't need to change culture.

instead religion can teach generally acceptable moral tenets and focus on solving actual social problems and then introduce the calendar as a practical solution that gets adopted because it's convenient.

names for days and months can be taken from local culture too.

not much contact is needed for that to happen. the religion will spread out from one location, but only a few people are enough to bring it to a new location. if the teachings are actually helpful, it will be accepted and within a few hundred years it can cover the whole planet.

if the new calendar is a significant improvement over any old calendar then not even a majority acceptance of the religion is needed. (we are still skeptical about your teachings, but your calendar is good, so we'll take that) (this is probably what allowed the gregorian calendar to be easily adopted in asia, and especially china, because it is not in itself culturally invasive)

worth reading is this section on why the julian calendar was created: it allowed for more stable dates of important events.

  • $\begingroup$ This was my first thought. $\endgroup$ Sep 13 '18 at 8:26

The very simplest reason: All other calendars sucked until a genius came up with a real good calendar. This calendar was so superior to its opposing ones that people's inertia, patriotism etc. was insufficient to cling to the other ones. So it conquered the world.

A good calendar is not easy. Most natural phenomena have odd ratios to each other. The moon here behaves very irregular (It was not until Euler came that it could be predicted quite accurately) and on average a moon cycle lasts 29.53059 days and a year is 365.2422 days. You also want it to accurately model seasons and use mostly regular intervals to allow fair pay for employers and easy usage of statisticians. (The 28 day February causes all kind of statistic problems).

Even the gregorian calendar (365.2425) is not the best available; in terms of accuracy (from best to worst) there is the Persian calendar, the Milancovic calendar (365.242222) and the Jewish calendar which is not so accurate with the sun year, but the very best as a combined moon/sun calendar.


No moon or no axial tilt

The easiest way to make all calendars the same is to have them all be as simple as possible

The Lunar and Solar calendars don't line up. Therefore, civilizations are constantly torn between following one or the other. The months we have on our calendar are mostly based on the lunar cycles; the years on the solar cycle.

Furthermore, axial tilt causes seasons and seasons can cause interesting events to line up with certain astronomical phenomena over the world. For example, during Ancient Egyptian times, Sirius rose above the horizon right before the Nile flood started. So the calendar there gave prominence to that event.

If you have a planet with no moon and no axial tilt, then you remove most of the variation that causes calendars to be different. There will only be two astronomical phenomena of note: days caused rising and setting of the sun, and years caused by the rotation of the stars through the sky. All calendars will have the same length of days and years, with no months. For added simplicity, make the length of the year an even integer multiple of the length of a day, so that the number of days per year doesn't change over time (at last, within one person's lifetime).

Now just line up the calendar's New Year

The only problem left is that the 'start' of the calendar years must be aligned with something. If you don't care about that, then you no problem. If you do care about then you need an astronomical event of notice that occurs at the same time every year.

My proposal would be a two-year event. Have the planet and another planet be in a 1:2 resonance. Every two orbits of the inner (inhabited) planet, there will be one orbit of the outer planet. The point of conjunction would be the most obvious starting point for a New Year. Then each cultural 'year' will actually be two full orbits of the sun (how ever many days that is), starting with the conjunction with the outer planet in resonant orbit.

  • 1
    $\begingroup$ I think even without the moon, people will want a way to sub-divide the calendar into more workable chunks, and without the moon I'd think those chunks would be more variable not less. $\endgroup$ Sep 13 '18 at 8:28

If you subscribe to evolution; the first intelligent beings arose somewhere, in a single tribe, due to some genetic mutation (in a male or female) that was inherited by the original's children.

That one person of modern intelligence would likely have had an enormous advantage over his tribe mates, likely enough to outsmart them and rule them all.

So this first King, living a long life, invented the calendar based on star positions, and passed that along to his intelligent children, that has passed it along for all time. Eventually the intelligent ones take over the earth and fragment into arguments; their shared ancestor in the distant past is no barrier to that.

But, they have all stuck with the original calendar: Every group has a correct legend that it was created by one of their own; it would be that first mutant with intelligence.

No conquering done to unite them here; this was fragmentation of the first family, after some generations and disagreements when their numbers grew large.

  • 2
    $\begingroup$ I see, but it's very unlikely that the first few barely-intelligent beings still struggling to communicate properly, could forge something so advanced. I mean it's something i'll think about but it's still a long shot $\endgroup$
    – Bolza
    Sep 12 '18 at 21:43
  • $\begingroup$ That's not how it works, intelligence isn't gradual. Somebody is the FIRST person just as intelligent as anybody living today. That person, by dint of intelligence, anticipation and outsmarting everybody else, mates frequently and young, creating a large family of children just as smart. Language must be invented, sure, but that is not the ONLY thing they invent. They will likely live together for a few centuries and exponentially invent things, because their numbers can grow enormously in that time. In our case, early inventions (language, farming, religion) are still with us today. $\endgroup$
    – Amadeus
    Sep 12 '18 at 21:57
  • $\begingroup$ I don't see how "subscribing" to evolution or how a relatively intelligent person tens or scores of thousands of years ago answers the question. Calendars won't arise until the need for them appears. By then, people are already spread all around the planet. Those that need calendars developed them; those that don't, well, they didn't. $\endgroup$
    – elemtilas
    Sep 12 '18 at 22:24
  • $\begingroup$ @elemtilas Those that don't believe in evolution will not necessarily believe in this route, of one smart person arising amongst many not-smart persons. Second, the need for calendars is pretty immediate if one is inventing the first human society in a span of a few centuries; first recognizing the patterns of seasons, rains, dry spells, developing farming, etc. Again, one family together in one spot. We have dozens of neolithic sites that are, in fact, very accurate astronomical calendars. $\endgroup$
    – Amadeus
    Sep 12 '18 at 22:31
  • $\begingroup$ I wonder if those same people will jump off a bridge, claiming not to "believe in" gravity. shrugs I could be wrong but I did not read in the OP's query that they're quickly inventing anything. Just that the societies within this world are very fragmented. Obviously, calendars in that world arose long before anyone felt the need for a common calendar. The query therefore isn't how calendars were invented, but rather, how did a single common calendar come about. $\endgroup$
    – elemtilas
    Sep 12 '18 at 22:40

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